"We have been working carefully towards an appointment that would ensure the board added operational experience at the highest level in the New Zealand energy industry," Flacks said.
Mercury is entering a development phase, with the construction of its first wind farm and the potential for further development of assets currently held by Mercury and assets to be acquired from Tilt Renewables.
Chief executive Vince Hawksworth said Barnes had a deep understanding of New Zealand's energy markets and had experience in investing in large scale assets.
Barnes will be classed as an independent director for the purposes of the NZX Listing Rules
The appointment becomes effective in September at the company's annual meeting.
Separately, Mercury said in its quarterly operational report that its hydro generation the fourth quarter equalled the prior comparable period for the lowest level of hydro generation in company history, due to sustained dry hydrology in the Waikato catchment.
Geothermal generation fell from 675GWh a year ago to 590GWh in the latest quarter due to an unplanned outage at the Kawerau power station that started on June 7.
The station is currently expected to return to service this week.
